1. Field of the Invention
This invention relates to a circuit for detection of a Dual Tone Multiple Frequency (DTMF) signal and for coincidence timing (i.e. insuring that two valid tone signals are simultaneously present for a predetermined duration of time). More particularly it relates to improvements in such a circuit regarding its capability of resolution at the leading edge of a DTMF signal burst.
2. Description of the Prior Art
A tone receiver is an electronic device used in telecommunications systems to convert a (DTMF) signal into a multiple digit binary code. The format of a DTMF signal consists of high and low tone signal bands (697-941 Hz., and 1209-1633 Hz.). Each band carries a selected one tone of a predetermined set of four tone signals unique to the respective band. That is, there is simultaneous presence of two tones, with one in each band, and with each such one tone in each band being a selected one of four possible tones for the band. The DTMF signal system is sometimes referred to as the "2-of-8" signal system.
Under U.S. telecommunication practices, a DTMF signal burst must be rejected if less than 20 milliseconds in duration, but must be detected if it is of 40 milliseconds duration or longer. A common practice is to design a tone receiver to detect signal bursts which are 30 milliseconds in duration or longer. The importance of good resolution at the leading edge of a DTMF signal burst and of minimizing the delay in initiating the coincidence timer for measuring the duration of the burst is apparent. Stated another way, it is important that as much of signal burst as possible be available for timing by the duration timer.
In one specific, it is important for a change in DTMF signal to be recognized in the situation of an occurrence without interruption of a shift from one valid DTMF signal to another valid DTMF signal. Also, because of the tendency of the selective filters (which are components of the tone detectors) to "ring", another very similar situation must be addressed. That is, it is important for the leading edge of a true DTMF signal to be recognized under the circumstances of such a false "ringing" signal followed immediately by a valid DTMF signal.
Accordingly, among the objects of the present invention are the provision of: claims 1 15, 0
1. A tone combination detector and coincidence timer circuit which provides improved resolution at the leading edge of a DTMF signal burst.
2. A circuit in accordance with the first enumerated object, which is further capable of recognizing a change in DTMF signal under the circumstances of an occurrence without interruption of a shift from one valid signal to another.
3. A circuit in accordance with the enumerated object, which is further capable of recognizing the leading edge of a true valid DTMF signal burst under the circumstances of a false "ringing" of selective filter of the tone receiver followed immediately by a true valid DTMF signal.
4. A circuit in accordance with the first enumerated object, which causes a minimal degradation of time response of the coincidence timer.